In the seismological investigation of subsurface formations various types of seismic energy sources have been used. These sources impart energy to the formation which returns to and is measured by detectors, such as geophones, positioned at various locations near the surface. The detected energy provides lithological and other information about the formation.
Explosive charges are one well known source of such energy. The use of explosive charges is often undesirable due to environmental concerns, safety concerns, storage requirements, and regulations in sensitive and populated areas. As a result, alternate energy sources have been developed which impart controlled vibratory signals to the earth formation.
The most common alternate energy source is a vibrator assembly carried by a land vehicle. The vibrator assembly includes a baseplate which is engaged with the ground. A reaction mass is supported above the baseplate. A vibrating driving force is applied between the baseplate and the reaction mass. The driving force is varied to produce either compressional waves or shear waves in the formation. The baseplate is loaded with the weight of the land vehicle and the reaction mass so that when the driving force is applied to the baseplate it will remain engaged with the ground.
Existing vibrator assemblies have a variety of shortcomings, one of which is related to the structure of the baseplate. Typically, a plurality of cleats project down from the baseplate. The cleats engage the ground and enhance the transfer of seismic energy to the formation. As a result of the large compressional forces placed on the baseplate during the operation of the vibratory source, ice, snow, clay, gravel, mud and other earth materials frequently lodge between the cleats. When earth materials are lodged between the cleats, the baseplate can not fully penetrate the earth surface. As a result, a poor vibrator-earth coupling is obtained, preventing the efficient transfer of vibrational energy to the formation. To assure proper operation when the ground is covered with snow and ice many times workmen must chip snow and ice from between the cleats each time the baseplate is lifted from the earth. This delays and increases the expense of the seismic survey.
It would be advantageous if an apparatus dislodged earth materials from between the cleats as the baseplate is lifted from the earth. It would be further advantageous if the apparatus was simply constructed, effective and reliable, particularly in an arctic environment.